The recent increase in the frequency of signals handled by electronic circuits has generally increased the importance of input protection circuits having excellent high-frequency characteristics. An input protection circuit is intended for preventing an electronic circuit from being damaged by an electrostatic discharge (ESD), which may occur in the process of handling the electronic circuit. An ESD is a phenomenon in which electric charges that have accumulated in the human body flow into the entrance (i.e. the input circuit) of the electronic circuit through air discharge or direct contact; this flow of electric charges generates a high voltage in the electronic components arranged in the input circuit and damages them. An electric discharge may also result from a rise in the voltage on the surface of an insulator due to the accumulation of electric charges caused by the action of an air current created, for example, by a fan for cooling an electronic device.
In a time-of-flight mass spectrometer or similar apparatus, a high-speed analogue signal is produced by an ion detector provided within an analyzer section using a high voltage under vacuum, and then fed into a waveform-recording circuit through a coaxial cable or the like (for example, refer to Patent Document 1). In such an apparatus, if an electric discharge occurs within the vacuum space, a high-voltage pulse is induced in the path of the analogue signal and reaches the waveform-recording circuit. Preventing this high-voltage pulse from reaching and damaging the input circuit is also an objective of providing the input protection circuit at the entrance of the waveform-recording circuit.
As shown in FIG. 1, a conventional input protection circuit uses a semiconductor ESD protection element for the purpose of input protection. The semiconductor ESD protection element 13 is provided in the signal path extending from an input connector 11 to an input circuit 12. In the case of measuring a high-speed analogue signal, an input resistor 14 having a resistance value equal to the characteristic impedance of the signal path is provided as a terminator at the entrance of the input circuit 12. The semiconductor ESD protection element 13 internally includes high-speed, low-capacity diodes, each diode having one end connected to the signal path and the other to a power source or ground. In the example of FIG. 1, one diode is connected to a clamping power line +VCL; if a high-voltage pulse with a positive polarity has entered the signal path from the input connector 11, this diode will turn to the conducting state and absorb the energy of the high-voltage pulse into the clamping power line +VCL, thus preventing the high-voltage pulse from reaching the input circuit 12. The example of FIG. 1 shows another diode, which is connected to another clamping power line −VCL and similarly prevents a high-voltage pulse with a negative polarity from reaching the input circuit 12. In order to direct the electric current resulting from the high-voltage pulse into the clamping power lines, the voltage of the signal path rises to a voltage that equals the voltage obtained by adding a forward voltage drop of the diode to the clamping power line +VCL or −VCL. However, the resultant voltage will not be so high as to damage the input circuit 12. One or both of the destinations of the semiconductor ESD protection element 13 may be connected to the ground in place of the clamping power line if the amplitude of the analogue signal to be handled is small or the signal is unipolar. For example, in the case of handling analogue signals ranging from 0 to 5 V, the ends of the semiconductor ESD protection element 13 are connected to a 5 V source and the ground, respectively.
As just described, the ESD protection circuit using a semiconductor ESD protection element shown in FIG. 1 can confine the voltage of the signal path within a specific voltage range and thus satisfactorily functions as the protector for the input circuit 12. However, the recent increase in the frequency band of the analogue signals to be handled has caused new problems, i.e. the distortion of the waveform of the analogue signal and the reflection of the analogue signal, due to the capacitance of the semiconductor ESD protection element 13.
FIG. 2 shows the result of a simulation of the distortion and reflection of a waveform in the case of feeding a triangular pulse with a rise time of 200 ps, a fall time of 200 ps and a peak height of 1 V into a waveform-recording circuit through a coaxial cable (delay time: 0.5 ns) having a characteristic impedance of 50Ω. Under the condition that the input circuit was terminated by an input resistor of 50Ω the calculation was performed for each of the semiconductor ESD protection elements having the capacitances 0 pF, 1 pF, 2 pF and 3 pF, and the calculated results are indicated by the square, rhombic, inverted triangular, and triangular plots, respectively. The triangular wave located in the lower left section of FIG. 2 is the pulsed voltage that was originally fed into the coaxial cable. The upper section of the same figure shows the voltage waveform created in the input circuit by a 0.5 ns-delayed arrival of the pulse at the waveform-recording circuit. Located in the lower right section of the same figure is the waveform of a reflected wave, which is returned to the transmitting end through the coaxial cable with an additionally delay of 0.5 ns.
In the case where the capacitance is 0 pF, the triangular wave that has entered the input circuit maintains its original form, and no reflection takes place. However, as the capacitance of the semiconductor ESD protection element increases, the increase in the voltage of the input circuit at the rising portion of the triangular wave becomes more delayed. As shown, even after the increasing rate of the voltage has reached a constant value, the voltage of the input circuit is lower than that of the input pulse, and this difference in the voltage is returned to the transmitting end as a reflected wave. An arrival of the input pulse at the waveform-recording circuit does not immediately cause an increase in the voltage of the input circuit since the pulse is initially used to charge up the capacitance of the semiconductor ESD protection element. For pulses at high frequencies, the input is short-circuited by the capacitance of the semiconductor ESD protection element, so that a reflected wave results. The difference between the voltage of the input pulse and that of the input circuit produces an electric current, which is used to charge the capacitance of the semiconductor ESD protection element. After the steady state is reached, the voltage of the input circuit begins to follow the voltage of the incoming pulse. In the case where the capacitance is 1 pF, the current required for charging the capacitance is 1V×1 pF÷200 ps=5 mA. This current is supplied from the 50Ω input resistor and the 50Ω coaxial cable, 2.5 mA each. Accordingly, the voltage appearing in the input circuit is 125 mV lower than that of the pulse and, simultaneously, a reflected wave with an amplitude of −125 mV results.
The decrease in the voltage becomes larger as the peak voltage of the pulse becomes higher. Furthermore, unlike the triangular wave, the waveform that is actually measured by the waveform-recording circuit does not have a constant slope. Therefore, the difference between the voltage of the input pulse and the voltage appearing in the input circuit changes with time, which causes a distortion or delay of the waveform.
The widespread use of high-speed digital communication techniques such as a universal serial bus (USB) or Ethernet in recent years has opened up more opportunities of using low-capacitance ESD protection elements that do not affect the device performance at high frequencies. These elements are called the polymer ESD protection element. This device includes a polymer film, which breaks down and turns to the conducting state when a high voltage is applied to it, thereby absorbing the energy of the electrostatic discharge into the ground or power source. The capacitance of the polymer ESD protection element can be decreased to as low as 0.1 pF, thus making it possible to reduce the distortion or delay of the input-pulse waveform to an allowable level. However, polymers differ from semiconductors in that they normally behave as insulators; turning such a material to the conducting state by breakdown requires a trigger voltage of 100 V or higher to be temporarily applied to it. An input circuit designed for handling signals at high frequencies has a rather low withstand voltage and hence is vulnerable to the application of an overvoltage. Accordingly, it is necessary to provide an additional protection element between the polymer ESD protection element and the input circuit.
An example of the ESD protection circuit using a polymer ESD protection element is shown in FIG. 3. As shown, the polymer ESD protection element 35 is connected to an intermediate point of the signal path extending from the input connector 31 to the input circuit 32. Polymer ESD protection elements are normally a bipolar device and should have the other end connected to the ground. As already explained, a high impedance element 36 is additionally provided between the polymer ESD protection element 35 and the input circuit 32 to prevent the trigger voltage from being directly applied to the input circuit 32 and damaging the input elements. In the USB or other interfaces, due to the signal input in a differential form, a common mode choke coil, filter element or the like is used as the high impedance element 36 in order to obtain a high impedance against the discharge energy while maintaining the high-frequency characteristic.
However, in the case of analogue signals, it is often impossible to adopt the differential signaling scheme. Using a resistor, coil or the like as the high impedance element 36 causes mismatching in the impedance of the signal line and hence a reflection of the signal. Furthermore, since analogue signals to be measured have a broad frequency band, it is impossible to use a filter having such a narrowband that matches the transmission frequency as in the case of the digital communication.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-32207